Chairs are widely used in daily life. While foldable chairs have the advantage of being folded so as to occupy a small volume, thus, foldable chairs are used more and more widely. FIG. 1 shows a general used foldable chair. The front supporting leg 10 is pivotally connected to the seat 40 by a pin 11. The rear supporting leg 20 is pivotally connected to the positioning confining piece 50 which is pivotally to the seat 40. The lower end of the positioning confining piece 50 has a concave groove 51. The seat 40 is installed with a positioning confining pin 41 which may be buckled into the groove 51 of the positioning confining piece 50. When the foldable chair is expanded, the seat 40 is pressed downwards, so that the pin 11 rotates around the fulcrum. When the groove 51 of the positioning confining piece 50 is buckled into the positioning confining pin 41, the foldable chair is expanded. Since in the prior art, the force is mostly suffered by the front supporting legs 10, the upward force can not be applied between the rear end of the seat 40 and the pin 11. Therefore, if a slightly force is applied to the rear end of the seat 40, the seat 40 will rotate backwards so that the chair is folded. The chair back 61 has often a flat plate. Thus the rear place of the seat 40 can not be used as a user sits on the seat 40, and he (or she) can not lean on the chair back. Moreover, since the pin 11 serves as a fulcrum. The rear supporting legs 20 has no structure for bearing the pressure of seat 40. Thus, the whole force is suffered by pin 11, and therefore, the pin 11 is easily destroyed.
FIG. 2 shows another prior art foldable chair. The rear supporting legs 20 are against the higher position of the front supporting legs 10 so to bear the downward force of the chair back. A further connected piece 22 serves to connect the rear supporting leg with the front supporting leg for avoiding the deformation induced by the distal end of the seat 40. Therefore, the user may lean against the chair back 62 so that the seat 40 will not be turned down or folded. The chair back 64 may have a cambered shape. However, in order to be installed with the connected piece 22, the joint of the front supporting leg and the rear supporting leg is higher. Moreover, since the force equilibrium position of the connecting piece 22 and the rear supporting leg 20 is higher. In order that the chair may be folded successfully, thus the height of the chair back 62 is higher. Although when the chair is expanded, the seat 40 would not be affected, however, after folded, the whole height is higher, thus the packaging volume is increased (referring to FIGS. 5A and 5B). This has a bad effect for carrying or storing.